The ZU civil registration was when they were used to watch over the 1994 elections. They were fitted with radar transponders (the object on the nose?) and filed IFR flight plans with the civil ATC authorities.

Thanks for the pics Spice! Are you able to clear up the different aircraft variants?

The thing on the nose is the Avitronics Airborne Emitter Location System, commonly referred to as the ELS. The system was later upgraded and with Avitronics now being owned by Saab the system is called the Electronic Surveillance Payload (ESP) which combines the ELS's functions with an improved Probability of Intercept for search radars.

The purpose of the ELS/ESP payload is to detect search, tracking and fire-control radars across the entire band from 0.5 GHz to 18 GHz, detect the location and type of each of these emitters and downlink the data to the ground station. This allows the operator to build up a an accurate picture of which enemy radars are currently turned on and emitting.

It's a very neat system, which I believe is in service with at least one Middle Eastern or North African country.